Contemporary combustion engines operate by burning a mixture of a hydrocarbon fuel and air in one or more piston cylinders to generate mechanical work. The engines beneficially operate with combustion in the one or more cylinders occurring at a high combustion temperature for achieving more efficient engine operation. However, engine operation at high combustion temperatures has associated therewith generation of soot and nitrogen oxide exhaust products, for example nitrous oxides (NOx). Soot and nitrogen oxide exhaust products are hazardous materials when ejected into the environment. A contemporary approach is thus to treat such exhaust products in apparatus included in exhaust systems of combustion engines.
For reducing concentrations of exhaust pollutants to within limits defined by legislation, for example contemporary European legislation pertaining to exhaust emissions, vehicle manufacturers conventionally employ Selective Catalytic Reduction (SCR) and/or Exhaust Gas Recirculation (EGR) in their vehicles. SCR is effective for absorbing nitrous oxides whereas EGR ensures more complete oxidation of nitrogen oxides present in exhaust gases.
SCR operation is improved by using chemical agents which are transported on vehicles. For example, “AdBlue” is a trade name for a chemical agent corresponding substantially to aqueous urea solution 32.5%. This solution is injected into exhaust gases of modern diesel engines during a post-combustion process for treating engine exhaust gases for reducing a proportion of harmful nitrous oxide (NOx) present in these gases. AdBlue is only ever used in conjunction with an SCR. Moreover, vehicles which are equipped with an SCR will carry an AdBlue storage tank in addition to a fuel tank. In operation, AdBlue is transferred from the storage tank and injected under pressure into exhaust gases where a series of chemical reactions occur as outlined in Table 1 below.
In FIG. 1, an exhaust system for a combustion engine 15 is indicated generally by 10. The system 10 includes in sequence an inlet 20 for receiving combustion gases from the engine 15, an oxidation catalyst 30, an AdBlue injection region 40, a hydrolysis catalyst 50, an SCR catalyst 60, an oxidation catalyst 70 and finally an outlet 80.
TABLE 1RegionReactionOxidation catalyst 302NO + O2 = 2NO22CO + O2 = 2CO24HC + 3O2 = 2CO2 + 2H2OHydrolysis catalyst 50CO(NH2)2 + H2O = 2NH3 + CO2SCR catalyst 608NH3 + 6NO2 + 7N2 + 12H2O4NH3 + 4NO = O2 + 4N2 + 6H2O2NH3 + NO + NO2 = 2N2 + 3H2OOxidation catalyst 704NH3 + 3O2 = 2N2 + 6H2O
Essentially, the process occurring in the exhaust system 10 involves an AdBlue mixture being passed onto a super-heated porous ceramic head of the SCR catalyst 60 whereat the de-ionised water evaporates and remaining urea of the injected AdBlue is passed on as a reagent which is operable to break nitrous oxide components (NOx) into mostly Nitrogen and water. Any contaminants present in the AdBlue will collect on the SCR catalyst 60, eventually causing it to clog and fail. It is therefore imperative that the AdBlue remains free from impurities through all stages of production, storage and dispensing.
Efficient AdBlue transfer relies on suitable equipment being installed onto vehicles. Failure of such equipment potentially results in premature product failure, or even worse a catastrophic event such as a complete SCR failure. AdBlue is very susceptible to contamination from both foreign matter and incorrect material selection. A main influence concerns the de-ionised water element of the solution which draws ions from materials which it comes into contact with; this changes the chemical composition of the AdBlue and causes salts to form which in turn clog the ceramic head on the SCR catalyst 60. Most common causes of premature failure of the SCR catalyst 60 are typically either a result of the ingress of damaged pump parts being accidentally transferred into the AdBlue tank or as a result of incorrect material selection. Inert materials should thus always be used to handle AdBlue.
The range of AdBlue transfer equipment on a vehicle must be reliable under diverse operating conditions in order to avoid costly damage occurring to an associated engine exhaust system of the vehicle. In particular, AdBlue must be delivered from an AdBlue storage tank of a vehicle to an exhaust system of the engine in a suitable ratio of urea to water. A problem in practice is that urea can freeze out of the AdBlue at low temperatures and result in blockage and a too dilute AdBlue being provided to the exhaust system. The present invention seeks to address this problem.